N-phosphonomethylglycine, also known as glyphosate, is a highly effective and commercially important herbicide useful in controlling the growth of germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants. Glyphosate is typically applied in the form of an aqueous solution of a salt thereof as a post-emergent herbicide for the control of growth of monocotyledonous and dicotyledonous plant species. Herbicidal salts of glyphosate and use thereof are described, for example, in U.S. Pat. No. 4,405,531.
Various methods for the production of glyphosate are known. For example, it can be produced by oxidizing N-phosphonomethyliminodiacetic acid in the presence of a catalyst. A variey of catalysts are known to catalyze such oxidation reactions. Glyphosate can also be produced by first reacting formylphosphonic acid (FPA) with glycine to form a condensation product and then hydrogenating the condensation product, for example, as described in U.S. Pat. No. 4,568,432. The latter synthetic route for glyphosate is desirable because a relatively high yield can be obtained. However, one limitation of this process is that the starting material, formylphosphonic acid, is not commercially available, and the methods known in the art for producing it suffers from one or more limitations.
For example, formylphosphonic acid is produced as a by-product in the electrochemical oxidation of nitrilotris-(methylenephosphonic acid) or substituted iminodimethylenediphosphonic acids to a secondary amine. This reaction requires the addition of a strong hydrochloric acid solution which presents safety, environmental, and equipment corrosion problems. Electrochemical methods generally require an external power source and other equipment which typically have higher maintenance needs and costs than non-electrochemical reactions.
Another known process in which formylphosphonic acid is produced as a by-product involves oxidation of tertiary phosphonomethylamines by oxygen in the presence of an activated carbon catalyst to cleave a phosphonomethyl group and to produce a secondary amine. Formylphosphonic acid may be produced as an intermediate cleavage fragment, with the fragment undergoing hydrolysis in a second step to form formic acid and phosphorous acid. Since a secondary amine is produced in the process, its presence necessitates additional steps, such as separation or recycling, which may result in manufacturing complexities.
Therefore, there exists a need for a new method for producing formylphosphonic acid and related compounds in an effective manner.